As broadly known, an electronic device such as a notebook type personal computer PC and a PDA (Personal Digital Assistant) is contrived to be supplied with electric power from the secondary battery. A majority of this type of electronic devices displays the residual quantity of the secondary battery at a rate against a full capacity, and hence an operator can grasp the residual quantity of the secondary battery through the residual quantity rate.
The secondary battery, as disclosed in Non-Patent documents 1 through 3, has such a discharge characteristic that a discharge voltage decreases on a step-by-step basis as accumulated electric power is consumed in a state approximate to a full charge and in a low charge state and decreases sharply as the accumulated electric power is consumed in a state approximate to a full discharge. Further, as indicated by a graph representing a relationship between the discharge voltage and the discharge capacity in FIG. 6, the secondary battery also has a discharge characteristic that the discharge capacity and the discharge voltage become lower in a large current discharge state IH (A) than in a small current discharge state IL (A). This is because an internal resistance of the secondary battery is higher in the large current discharge state IH (A) than in the small current discharge state IL (A).
By the way, when the secondary battery excessively gets approximate to the full discharge, battery performance deteriorates to such an extent as to be disabled from recovering. Therefore, a discharge cutoff voltage value VCUTOFF (V) is determined as a limit to which given performance is kept in the secondary battery. A housing of the secondary battery is mounted with a protection circuit, which is contrived to monitor the discharge voltage value of the secondary battery and to, when the discharge voltage value becomes lower than the discharge cutoff voltage value VCUTOFF (V), stop discharging the electricity of the secondary battery (stops supplying the electric power to the device).
The residual quantity rate displayed by the electronic device described above is calculated based on a difference between the discharge voltage value and the discharge cutoff voltage value VCUTOFF (V). Hence, for example, in the small current discharge state IL (A), when the discharge voltage value slightly exceeds the discharge cutoff voltage value VCUTOFF V) (state SA in FIG. 6) and when the small current discharge state IL (A) continues as it is, a capacity remaining in the secondary battery can be used, and therefore the operator does not get confused even when the electronic device displays “Residual quantity still remains”.
When the discharge current rises with an increase in demand for the electric power with the result that the large current discharge state IH (A) occurs, however, the discharge voltage value decreases under the discharge cutoff voltage value VCUTOFF (V) (state SB in FIG. 6), and the protection circuit stops discharging, resulting in disabling the capacity remaining in the secondary battery from being used. At this time, the electronic device displays “Residual quantity still remains”, and nevertheless, since the supply of the electric power is abruptly stopped as the demand for the electric power rises, it follows that the operator gets confused.
Thus, the discharge cutoff voltage value VCUTOFF (V) is used as a threshold value, in which case it might happen that the residual quantity of the secondary battery is not properly displayed.
Further, Patent document 1 discloses that whether there is the residual quantity of the battery capacity is determined depending on whether a quantity of electric current consumption of the electronic device reaches a maximum capacity CMAX (Ah) that is predetermined with respect to the secondary battery. In this Patent document 1, the quantity of electric current consumption of the electronic device is obtained by taking a total sum of the quantities of electric current consumption of the respective units within the device. Further, the quantities of electric current consumption of the respective units are calculated by recording the current values when operating and when standing by as required and integrating the current values at the respective points of time. Moreover, in this Patent document 1, the maximum capacity CMAX (Ah) of the secondary battery is a capacity measured by actually discharging the electricity of the secondary battery.
The maximum capacity CMAX (Ah) predetermined with respect to the secondary battery, however, differs depending on whether the measurement is done in the large current discharge state IH(A) or in the small current discharge state IL(A). Therefore, for instance, the maximum capacity CMAX (Ah) is measured in the large current discharge state IH (A), in which case a capacity smaller than the capacity actually remaining in the secondary battery is displayed as the residual quantity when the quantity of the electric current consumption of the electronic device is a quantity accumulated in the small current discharge state IL(A), and it follows that the quantity of the electric current consumption reaches the maximum capacity CMAX (Ah) and the supply of the electric power is thereby stopped irrespective of the usable residual quantity being left in the secondary battery. Further, e.g., in the case where the maximum capacity is the capacity measured in the small current discharge state IL(A), when the quantity of the electric current consumption of the electronic device is the quantity accumulated mainly in the large current discharge state IH (A) the capacity larger than the capacity, which is actually left in the secondary battery, is displayed as the residual quantity, and “Residual quantity still remains” might be displayed even when the discharge voltage is under the discharge cutoff voltage value VCUTOFF.
Thus, even in the case of using the maximum capacity CMAX (Ah) as the threshold value, the residual quantity of the secondary battery is not properly displayed as the case may be.